Will I be sinful for the accident I was involved in, knowing that I did not know that the driver had stolen the car and did not have a driver's license? What should I do now? And what do you advise me to do?

The questioner should have avoided riding with the reckless driver and gotten out, but he did not. He must be careful not to associate with bad companions in the future. What happened was by Allah's divine decree and destiny, and it is an expiation for sins. A believer is rewarded for every calamity and harm that befalls him if he shows patience and seeks his reward from Allah. This incident should be the beginning of repentance for the questioner.